Senior US intelligence officials will testify in Congress on Thursday on Russia's alleged cyber-attacks during the 2016 election campaign, even as President-elect Donald Trump casts doubt on intelligence agencies' findings that Moscow orchestrated the hacks.

The hearings come a day before Trump is due to be briefed by intelligence agency chiefs on hacks that targeted the Democratic Party.

Trump is heading for a conflict over the issue with Democrats and fellow Republicans in Congress, many of whom are wary of Moscow and distrust the New York businessman's praise of Russian President Vladimir Putin and efforts to heal the rift between the US and Russia.

Director of National Intelligence James Clapper, National Security Agency Director Mike Rogers and Under-secretary of Defence for Intelligence Marcel Lettre are expected to appear before the Senate Armed Services Committee, which is chaired by Republican John McCain, a vocal critic of Putin.

Their testimony on cyber threats facing the US will come a week after President Barack Obama ordered the expulsion of 35 Russian suspected spies and imposed sanctions on two Russian intelligence agencies over their alleged involvement in hacking US political groups in the 2016 election.

US intelligence agencies say Russia was behind hacks into Democratic Party organisations and operatives before the presidential election, a conclusion supported by several private cyber-security firms. Moscow denies the hacking allegations.

US intelligence officials have also said the Russian cyber attacks aimed to help Trump defeat Democrat Hillary Clinton in the November 8 election. Several Republicans acknowledge Russian hacking during the election but have not linked it to an effort to help Trump win.

Documents stolen from the Democratic National Committee and John Podesta, Clinton's campaign manager, were leaked to the media in advance of the election, embarrassing the Clinton campaign.

In a tweet on Wednesday, Trump said, "(WikiLeaks founder) Julian Assange said 'a 14 year old could have hacked Podesta' - why was DNC so careless? Also said the Russians did not give him the info!" Trump also quoted Assange as telling Fox News that US media coverage of the matter was "very dishonest."

He and top advisers believe Democrats are trying to de-legitimise his election victory by accusing Russian authorities of helping him.

Firmer response urged

Some lawmakers, including McCain, said a firmer response was needed to check Russian aggression in cyberspace and elsewhere. He is among a handful of Republicans to join Democrats in pushing for a special committee to investigate Russia's political hacking, although that effort has lost traction in the face of opposition from Republican leaders in Congress.

Obama instructed US intelligence agencies last month to conduct a full review of the election hacks. That review could be completed and delivered to Obama as soon as Thursday, said sources familiar with the matter.

Five Democratic senators introduced legislation on Wednesday calling for the creation of an independent, non-partisan commission to investigate Russian interference in the election.

Trump has also nominated people seen as friendly toward Moscow to senior administration posts, including secretary of state nominee Rex Tillerson, who while Exxon Mobil chief executive, was awarded the Order of Friendship, a Russian state honour, by Putin in 2013.

Rogers, the NSA chief, visited the president-elect in New York in November and is among a handful of people being considered by Trump to succeed the retiring Clapper as US spy chief, in addition to former Republican Senator Dan Coats, according to sources familiar with the matter.

The Senate Foreign Relations Committee will also hold a closed-door hearing on Thursday to examine Russia's alleged hacking and harassment of US diplomats.